code: def test_substitute(self):
      data = AudioLoader.load_mel_spectrogram(
          self.sample_wav_file, n_mels=128
      )
      aug = nas.LoudnessAug(stateless=False)
  
      aug_data = aug.augment(data)
      aug_audio = aug_data[0]
  
      comparison = (
          data[:, aug.time_start : aug.time_end]
          == aug_audio[:, aug.time_start : aug.time_end]
      )
      self.assertFalse(comparison.all())
      comparison = (
          data[:, : aug.time_start] == aug_audio[:, : aug.time_start]
      )
      self.assertTrue(comparison.all())
      comparison = (
          data[:, aug.time_end :] == aug_audio[:, aug.time_end :]
      )
      self.assertTrue(comparison.all())
  
file_path: nlpaug/test/augmenter/spectrogram/test_loudness_spec.py
imports: import unittest
  import os
  import numpy
  from dotenv import load_dotenv
  from nlpaug.util import AudioLoader
  import nlpaug.augmenter.spectrogram
label_desc: Use image node_id 3 for calling the TestLoudnessSpec obj's underlying member method code with example usage: obj.test_substitute() without return types